Eukaryotic genes consist of a transcription/translation initiation region, a coding region and a termination region. The transcription/translation initiation region is typically located upstream of the coding region, or in other words, entirely to the 5' terminal end of the coding region. This initiation region includes a "promoter" region, the element responsible for inducing transcription and "untranslated sequences" responsible for binding to ribosomes and translation initiation. The translation-related regions of these "upstream" regulatory sequences are sometimes referred to as the "mRNA untranslated leader." They vary in length and base composition from gene to gene.
The characteristics of the promoter will determine the level and timing of transcription. A promoter typically consists of a "TATA box" and an "upstream activating region" (sometimes referred to herein as "UAR"). The TATA box is responsible for marking the initiation of transcription approximately -25 or 25 base pairs in the 3' direction toward the start of the coding region. Through recombinant techniques, a plant transcription/translation initiation region can be designed to activate a nucleic acid sequence of interest, such as a DNA sequence encoding a heterologous or non-naturally occurring gene, in a plant host cell. And by modifying the promoter region of a construct capable of expression in a plant host cell, timing and the level of expression of transcription can be controlled.
As noted above, recombinant DNA technology is now being applied to plants. Researchers are able to modify plant genetic material and achieve expression of proteins of interest in a plant host cell, for example. However, it is often desired to increase the expression of the nucleic acid sequence of interest. Higher levels of expression may be desired to increase the level of the desired protein in the consumer product, to have a desired enzymatic or other effect on a plant cell biochemical pathway, to create more anti-sense copies of an endogenous gene thereby reducing the amount of mRNA transcript which could then be processed by the cell, or the like. Methods to achieve increased plant cell expression include the search for stronger promoters, gene amplification, and use of enhancer regions to boost the expression level of weaker promoters.